[Aramant]

Tieflings are actually NOT a sub-class of elf. Whoever it was that said the thing about them being partly demonic was correct. Tieflings (and Aasimars) are "Planetouched". The actual D&D descriptions is as follows:

"Planetouched" is a general word used to describe those who can trace their bloodline back to an outsider, usually a fiend or celestial.
The effects of having a supernatural being in one's heritage last many generations. Although not as dramatically altered as a half-celestial or half-fiend, the planetouched still retain some of their special qualities.
The planetouched discussed here are the most common. Aasimars are humans with some trace celestial blood in their veins, and tieflings have some fiendishness in their family tree.
There is no "typical" assimar or tiefling. They do not have their own societies or cultures, instead blending into existing ones.

(Aasimar entry omitted)

Tiefling:

Twisted, devious, and untrustworthy, tieflings more often than not follow their inherent traits and heed the call to evil. A few defy their nature, but still must fight against popular opinion (if their nature is known) or the feeling of otherworldly "wrongness" that seems to follow them wherever they go.
Beyond this aura that many find disturbing, many tieflings are indistinguishable from humans. Others have small horns, pointed teeth, red eyes, a whiff of brimstone about them, or even cloven feet. No two tieflings are the same.
In most human societies, tieflings maintain a low profile, operating as thieves, assassins, or spies. Occasionally they rise to a position of power, but when their true nature is revealed they quickly become outcasts.